Electronic equipment working in the sea or other saline waters, such as marine aids to navigation, hydro-meteorological buoys, underwater detectors, unlimited sensors and so on, need batteries to provide energy as power sources. The electrical equipment above requires battery with higher capacity and better stability. Conventional primary batteries, such as zinc-manganese, zinc-silver etc., have a higher price, but lower quantity/volume energy density and worse storage performance. If lead-acid, nickel metal hydride, or lithium ion secondary battery is used, on the one hand, the continuous using time underwater is limited by the rated capacity; on the other hand, it needs to be sealed in a pressure-resistant container while working in the deep sea, especially for the lithium-ion battery, which further increases the complexity of the system.
Seawater battery of dissolved oxygen is a kind of battery used underwater, which selects metal, such as magnesium, aluminum or the alloy as the anode reactant, oxygen as the oxidant, and seawater or saline water as the electrolyte. The advantages of such batteries are:
1. High energy density. As both of the oxidant and the electrolyte, except the metal anode, are provided by seawater directly, the theoretical energy density in quantity is up to hundreds of watts per kilogram.
2. Rich source of raw materials. Both magnesium and aluminum are rich in earth and low in price.
3. Perfect storage performance. Such battery is inactive when isolated with seawater or saline water. Thus it has a perfect storage performance with a period of time up to several years. However, the dissolved oxygen in seawater is used as oxidant in such a battery and its content is too low (only 0.3 mol/m3), the cathode must be an open structure to ensure its contact with seawater. At the meanwhile, the enlarged cathode area can contribute a relatively large current for such battery.
In order to expand the cathode area of seawater battery, patent CN1543001A disclosed a divergent cathode structure with a magnesium rod in its central. But such structure can't guarantee the smooth flow of seawater throughout the internal battery completely, and the distance between the magnesium anode and the cathode is a little far, which increases the ionic resistance and further lowers the battery performance. Courtesy of Westinghouse Corporation in US disclosed that a circle cathode was placed around the cylindrical periphery of the magnesium anode to ensure the cathode full contact with seawater, while the problem of poor seawater circulation also exists in such structure. Besides, the volume of such battery is too large and the internal space is not fully utilized.
On the whole, the problems, such as small effective area of the electrode, low effective capacity, poor internal flow of seawater, lack of oxygen supply, being wound and polluted by the seawater bliss algae easily, etc., haven't been solved systematically for the battery systems used under seawater. Thus, an effective solution to the above problems is critical to the further application of such battery system used under seawater.